1. Field of the Invention
This invention relates to novel nematic liquid crystal compositions having electro-optical properties which change their optical characteristics in response to an electric field applied thereto.
2. Description of the Prior Art
It is well known that certain nematic liquid crystal materials show an electro-optical scattering effect according to the voltage applied. These materials are widely used in electro-optical devices, such as alpha-numeric displays and light-valves. Such devices are described in more detail in the U.S. Pat. No. 3,322,485 by Richard Williams and the U.S. Pat. No. 3,499,112 by George H. Heilmeier et al.
The useful temperature range of such electro-optical devices employing nematic liquid crystals is limited by the temperature range over which the nematic liquid crystal material filled in the interior exhibits the nematic mesophase.
Known organic liquid crystal single compounds having the nematic mesophase temperature range around room temperature usually have an operable temperature differential of about 20.degree. C to 40.degree. C, which is extremely restrictive. For example, p-methoxybenzylidene-p'-n-butylaniline (hereinafter referred to as MBBA) has a nematic mesophase from 20.degree. C to 47.degree. C, while p-ethoxybenzylidene-p'-n-butylaniline (hereinafter referred to as EBBA), has the temperature range of 37.degree. C to 80.degree. C. Electro-optical devices employing the nematic liquid crystal are often desired to be used at normal ambient temperature from -20.degree. C to 80.degree. C, but such organic compounds as the MBBA, EBBA or other typical nematic liquid crystal materials are not operable over the entire range of desired temperatures.
Attempts have been made by prior workers for formulating a nematic liquid crystal composition possessing wider temperature range by mixing several kinds of nematic liquid crystal material. For instance, the nematic liquid crystal composition obtained by mixing MBBA and EBBA with the weight ratio of 2:1 produces a nematic liquid crystal in the temperature range from 15.degree. C to 60.degree. C. When the temperature is raised gradually from low temperature, the nematic liquid crystal composition including the MBBA and EBBA with the weight ratio of 2:1 changes from the solid phase to the nematic liquid crystal phase at 15.degree. C and further changes into the isotropic phase at 60.degree. C.
The transition temperature from the solid phase to nematic liquid crystal phase is called the S.fwdarw.N point. The temperature for transition from the nematic liquid crystal phase to the isotropic phase is called the N.fwdarw.I point. However, the phase changes do not reverse at the same temperatures. When the temperature is lowered gradually from a high temperature, the binary 2:1 MBBA:EBBA nematic liquid crystal composition changes into the nematic liquid crystal phase from the isotropic phase at 60.degree. C and further changes into the solid phase at -5.degree. C. These transition temperatures are also called the I.fwdarw.N point and the N.fwdarw.S point, respectively. In general the temperature for the S.fwdarw.N point is higher than the N.fwdarw.S point and the binary liquid crystal composition has a nematic liquid crystal phase within the effective temperature range from -5.degree. C to 60.degree. C. However, the liquid crystal composition is in a phase called the supercooling phase at a temperature from the N.fwdarw.S point at -5.degree. C to the S.fwdarw.N point at 15.degree. C and when some stimulation is given, the composition may crystallize into the solid phase. For example, when some external forces are imposed on the liquid crystal or when the purity of the liquid crystal composition differs, the liquid crystal phase itself under the supercooling phase becomes thermodynamically unstable, and the liquid crystal composition may fail to show the nematic mesophase properties under the said supercooling conditions. Therefore, the operable temperature range for electro-optic devices cannot be determined reliably.